Abstrakt
The residual strain in pure printed circuit boards (PCBs) and PCBs with Cu-plated holes has been obtained by measurement of the temperature dependence of their dilatational characteristics in the x, y, and z directions up to 240A degrees C. Shrinkage in all directions was observed for all samples of both materials in the first thermal cycle.
No permanent length changes were observed in the second or subsequent thermal cycles. The residual strain was determined from the difference in relative elongation between the first and second thermal cycles.
Relaxation of residual strain occurred only in the first thermal cycle, as a thermally activated process. The highest value of relaxed residual strain was found in the z direction for both materials.
Relaxation of residual strain in the z direction of the pure PCB occurred only in the negative strain range, whereas relaxation of the PCB with Cu-plated holes occurred in both the positive and negative strain ranges. The relaxation of the positive strain in the PCB with Cu-plated holes in the z direction implies that this part of the PCB was under pressure during its preparation.
This relaxation is a consequence of the high coefficient of thermal expansion of PCB laminate in this direction, which can also lead to cracks in Cu holes when the material is heated above the glass-transition temperature.